1. Field of the Invention:
This invention relates to electrochemical cells and it has particular relation to electrochemical cells which are responsive to a constituent of a reactive mixture.
The invention is particularly desirable for developing information from a solid electrolyte cell concerning oxygen present in a chemically reactive combustible mixture which may include a fuel such as methane. For this reason the invention will be described as applied to oxygen-responsive devices.
2. Description of the Prior Art:
Galvanic cells for measuring the oxygen content of gasses are well known. For example, cells using a hydroxide-ion conducting electrolyte, that consists of a fused alkali hydroxide or aqueous hydroxide, can be used to obtain an electric output responding to differences in oxygen concentration at two electrodes in the same electrolyte.
A particularly useful device utilizes the oxygen ion conductivity of certain solid oxides. Such solid electrolytes may consist of an oxide of a tetravalent element such as zirconium, thorium, or hafnium suitably doped with an oxide of elements of lower valency to impart a greater ionic conductivity to the tetravalent oxides. Reference is made to a patent of R. J. Ruka and J. Weissbart, U.S. Pat. No. 3,400,054. The electrolyte is bounded by two electronically conducting electrodes. The cathode, the positive pole of the cell, is in contact with the higher oxygen concentration. The reaction here is EQU O.sub.2 (p.sub.1) + 4 e.sup.- .fwdarw. 2O.sup.2.sup.-.
At the anode EQU 20.sup.2.sup.- .fwdarw. O.sub.2 (p.sub.2) + 4 e.sup.-, with pressure p.sub.1 &gt; p.sub.2.
The voltage E at open circuit, or when small enough currents are drawn from the cell, is given by the Nernst equation EQU E = 4.954 .times. 10.sup.-.sup.5 T log p.sub.1 /p.sub.2
Where T is the temperature on the absolute scale. P.sub.1 may be the reference pressure. For example, it can be supplied by air at atmospheric pressure or by the vapor pressure of an oxide applied to the electrode.
As described in the Hickam U.S. Pat. No. 3,347,767 such a cell may comprise a tubular electrolyte member of a solid material such as a calcium-stabilized zirconium-oxide electrolyte. Such a member may have a length of eight inches an inner diameter of 1/8 inch and an outer diameter of 3/16 inch. The tube is coated with inner and outer porous electrodes of platinum. When operated at a suitable temperature such as 400.degree.C to 1000.degree.C (850.degree.C is very satisfactory) such a cell develops an electrical output from the electrodes which is dependent on the ratio of the partial pressures of oxygen present at the two electrodes. Reference may also be made to the Hickam U.S. Pat. No. 3,404,836.